The neural crest is a transient embryonic precursor population, whose derivatives include cells having widely different morphologies, characteristics and functions. These derivatives include the neurons and glia of the entire peripheral nervous system, melanocytes, cartilage and connective tissue of the head and neck, stroma of various secretory glands and cells in the outflow tract of the heart (for review, see Anderson, D. J. 1989. Neuron 3:1-12). Much of the knowledge of the developmental potential and fate of neural crest cells comes from studies in arian systems. Fate maps have been established in aves and provide evidence that several different crest cell derivatives may originate from the same position along the neural tube (Le Dourain, N. M., 1980, Nature 286:663-669). Schwann cells, melanocytes and sensory and sympathetic neurons can all derive from the truncal region of the neural tube. On the other hand, some derivatives were found to originate from specific regions of the crest, e.g., enteric ganglia from the vagal and sacral regions. These studies also revealed that the developmental potential of the neural crest population at a given location along the neural tube is greater than its developmental fate. This suggests that the new environment encountered by the migrating crest cells influences their developmental fate.
Single-cell lineage analysis in vivo, as well as clonal analysis in vitro, have reportedly shown that early avain neural crest cells are multipotential during, or shortly after, their detachment and migration from the neural tube. In avian systems, certain clones derived from single neural crest cells in culture were reported to contain both catecholaminergic and pigmented cells (Sieber-Blum, M. and Cohen, A. M. 1980, Dev. Biol. 80:96-106). Baroffio, A. et al. (1988, Proc. Natl. Acad. Sci. USA 85:5325-5329) reported that avian neural crest cells from the cephalic region could generate clones which gave rise to highly heterogeneous progeny when grown on growth-arrested fibroblast feeder cell layers.
In vivo demonstration of the multipotency of early neural crest cells was reported in chickens by Bronner-Fraser, M. and Fraser, S., 1989, Neuron 3:755-766. Individual neural crest cells, prior to their migration from the neural tube, were injected with a fluorescent dye. After 48 hours, the clonal progeny of injected cells were found to reside in many or all of the locations to which neural crest cells migrate, including sensory and sympathetic ganglia, peripheral motor nerves and the skin. Phenotypic analysis of the labelled cells revealed that at least some neural crest cells are multipotent in vivo.
Following migration from the neural tube, these early multipotent crest cells become segregated into different sublineages, which generate restricted subsets of differentiated derivatives. The mechanisms whereby neural crest cells become restricted to the various sublineages are poorly understood. The fate of neural crest derivatives is known to be controlled in some way by the embryonic location in which their precursors come to reside (Le Douarin, N. M., 1982, The Neural Crest., Cambridge University Press, Cambridge, UK). The mechanism of specification for neural crest cells derivatives is not known. In culture studies described above, investigators reported that clones derived from primary neural crest cells exhibited a mixture of phenotypes (Sieber-Blum, M. and Cohen, A. M. 1980, ibid; Baroffio, A. et al., 1988, ibid; Cohen, A. M. and Konigsberg, I. R., 1975, Dev. Biol. 46:262-280; Dupin, E. et al., 1990, Proc. Natl. Acad. Sci. USA 87:1119-1123). Some clones contained only one differentiated cell type whereas other clones contained many or all of the assayable crest phenotypes.
The observation that apparently committed progenitors and multipotent cells coexist in the neural crest may be interpreted to reflect a pre-existing heterogeneity in the population of primary crest cells or it may reflect asynchrony in a population of cells that undergoes a progressive restriction in developmental potential. Given the uncertainty in the art concerning the developmental potential of neural crest cells, it is apparent that a need exists for the isolation of neural crest cells in clonal cultures. Although culture systems have been established which allow the growth and differentiation of isolated avian neural crest cells thereby permitting phenotypic identification of their progeny, culture conditions which allow the self-renewal of multipotent mammalian neural crest cells have not been reported. Such culture conditions are essential for the isolation of mammalian neural crest stem cells. Such stem cells are necessary in order to understand how multipotent neural crest cells become restricted to the various neural crest derivatives. In particular, culture conditions which allow the growth and self-renewal of mammalian neural crest stem cells are desirable so that the particulars of the development of these mammalian stem cells may be ascertained. This is desirable because a number of tumors of neural crest derivatives exist in mammals, particularly humans. Knowledge of mammalian neural crest stem cell development is therefore needed to understand these disorders in humans. Additionally, the ability to isolate and grow mammalian neural crest stem cells in vitro allows for the possibility of using said stem cells to treat peripheral neurological disorders in mammals, particularly humans.
Accordingly, it is an object herein to provide clonal cultures of mammalian neural crest stem cells and their progeny in feeder cell-independent cultures. An object of the invention is directed to the demonstration that multipotential stem cells exist in the neural crest. Another object of the invention is the demonstration that these neural crest stem cells have at least limited self regeneration capacity and undergo lineage restriction in a manner that is sensitive to the local environment. A further object of the invention is to provide methods which allow the growth and regeneration of neural crest stem cells in feeder cell-independent cultures. Another object of the invention is to provide methods which allow the differentiation of these neural crest stem cells into at least the progenitors for, as well as, more differentiated neurons and glia of the peripheral nervous system (PNS). A further object of the invention is to provide methods which allow for the identification of mammalian neural crest stem cells using transplantation assays. Still further, an object of the invention is to provide methods for transplanting neural crest stem cells or their progeny into a mammal.